1. Field of the Invention
The present invention generally relates to measuring pressure within an enclosed space by using a capacitance diaphragm gauge and, more particularly, is concerned with a highly sensitive capacitance diaphragm gauge which is configured to minimize inaccuracy in measurements resulting from mechanical vibrations and other effects of external forces applied to the gauge.
2. Description of the Prior Art
In a system having liquids or gasses that must be maintained within a predetermined pressure range, it is necessary to accurately measure the pressure. Presently, numerous devices are available to measure pressure. Some of these devices indicate pressure while others act as transducers by converting the measured pressure to a calibrated quantity to be transmitted to another system. For example, in a system which utilizes electronic circuits to automatically control the pressure, it is desirable to provide an electrical representation of the pressure that can be provided as an input to the electronic circuitry. One such device that is used to generate an electrical representation of the pressure is a capacitance manometer, or capacitance diaphragm gauge.
An exemplary capacitance manometer is described in U.S. Pat No. 3,557,621, issued on Jan. 26, 1971. Briefly, such a capacitance manometer has a diaphragm comprising an electrically conductive material that is supported along the periphery of the diaphragm by a housing or other such support structure. Typically, the housing is constructed from an electrically conductive metal so that the housing provides an electrically conductive path to the diaphragm. The diaphragm, which functions as one electrode of a capacitor, is positioned proximate to at least one reference electrode, which, as described in U.S. Pat. No. 3,557,621, is preferably fixed. For example, the fixed reference electrode is advantageously mounted On a ceramic disc substrate which is itself mounted to the housing.
One side of the diaphragm is exposed to a known or reference pressure and the other side of the diaphragm is exposed to an unknown, variable pressure that is to be measured. A differential in the pressure between the two sides of the diaphragm cause the center of the diaphragm to move in the direction of the lower of the two pressures and thus causes the diaphragm to develop a curvature. Such movement and resulting curvature causes the center of the diaphragm to move closer to or further from the fixed reference electrode of the capacitor and thus causes a corresponding change in the capacitance between the two electrodes. The capacitance between the two electrodes can be monitored, for example, by the circuit shown in FIG. 2 of U.S. Pat. No. 3,557,621, to thereby detect the movement of the diaphragm and thus detect changes in the pressure. The electrical output signal of the circuit can be measured by known devices and calculations performed on the measured signal value to provide an indication of the pressure differential.
As used herein, the full scale range of a capacitance manometer is the maximum pressure differential that the capacitance manometer can measure. Capacitance manometers of the type described above typically had a full scale range on the order of 1 torr. The sensitivity of a capacitance manometer is the smallest amount of pressure differential that can be measured by a capacitance manometer. Typically, the theoretical sensitivity of a capacitance manometer is a small fraction of the full scale pressure range. For example, the more recently developed capacitance manometers have sensitivities in the range of 10.sup.-6 or 10.sup.-7 the full scale range of the manometer. For example, a capacitance manometer having a full scale range of 1 torr theoretically could have a sensitivity of 0.1-1 micro-torr (1 micro-torr equals 1.times.10.sup.-6 torr). However, as discussed below, it has not been believed possible to achieve such sensitivity.
One problem which has previously prevented producing capacitance manometers with sensitivities in the theoretical range is that changes in the ambient temperature results in thermal expansion or contraction of the components used in the capacitance manometer. Specifically, the base supporting the fixed reference electrode is typically made of ceramic whereas the housing of the manometer by which the diaphragm, or variable reference electrode is supported, is typically made of metal. The metal and the ceramic have different thermal coefficients of expansion and contraction, consequently, they expand and contract to a different extent due to changes in the ambient temperature. This expansion and contraction of the housing and ceramic base often results in bending and curvature of the ceramic base forming the fixed reference electrode. Consequently, the capacitance measured by the manometer changes as a result of bending of the ceramic base. The changed capacitance is indistinguishable from a change in capacitance resulting from a change in pressure.
One solution to this problem was disclosed in U.S. Pat. No. 4,823,603, issued Apr. 25, 1989. This patent discloses using a roller bearing structure positioned between the housing and the ceramic base to minimize bending of the ceramic base and thereby minimize any changes in the capacitance read by the manometer resulting from thermal expansion and contraction of the components forming the manometer. Use of this roller bearing structure has allowed manufacturing of capacitance manometers having a full scale range on the order of 100 milli-torr, and having a corresponding potential sensitivity in the range of 0.01-0.1 micro-Torr; however, for reasons discussed below, the practical sensitivity under non-ideal operating conditions is around 10 micro-torr and does not come close to the potential sensitivity.
However, in some fields, such as semiconductor etching and manufacturing, there is a need for a pressure sensor which can detect changes of pressure on the order of 1 micro-torr or have sensitivities of approximately an order of magnitude greater than what is currently available. Unfortunately, pressure sensitivities in a range less than 10 micro-torr heretofore have been unattainable.
One reason which has precluded prior art capacitance manometers from having better than 10 micro-torr sensitivity is that the force exerted against the diaphragm resulting from the earth's gravitation or from inertial forces caused by vibration can, in some cases, approximate the force that the diaphragm would experience when subjected to a 10 milli-torr pressure differential, for example. Thus, such forces can cause the diaphragm to deflect sufficiently for the capacitance manometer to signal a pressure change when there was, in fact, no pressure change. Thus, although the manometer has a much better potential sensitivity, the ability to sense very small pressure differentials in the range of 1 micro-torr to 10 micro-torr is obscured by the effects of gravity or vibration.
A constant gravitational force on diaphragm does not by itself result in any inaccurate or false readings by the capacitance manometer because it produces a constant amount of deflection of the diaphragm and a constant capacitance reading which can be removed by calibration of the manometer. Consequently, the sensitivity of a stationary capacitance manometer to incremental changes in pressure is not usually affected by constant gravitational force. However, when vibrational forces are transmitted to the capacitance manometer or when the capacitance manometer is temporarily reoriented, the diaphragm deflects in the same manner that it would deflect as a result of a pressure differential. These vibrational forces can readily exceed 1 micro-torr, and, depending on the direction in which the vibrational forces are applied with respect to the diaphragm, may be as great as 10 milli-torr, or higher. Vibrational forces, for example, can be transmitted to the capacitance manometer as a result of the innumerable causes of vibration of the surface on which the capacitance manometer is mounted. For example, an individual walking by and bumping or brushing the structure on which the capacitance manometer is mounted, can result in false signals from the capacitance manometer. Further, vibrational forces sufficient to induce a false signal by the capacitance manometer can even be created by a large truck driving by on the street and shaking the building in which the capacitance manometer is being used.
Heretofore, there has not been any way to distinguish between the signals generated by the capacitance manometer as a result of changes in pressure less than 10 micro-torr and the signals generated by the capacitance manometer resulting from vibration induced non-constant gravitational forces. Consequently, prior art capacitance manometers of the type described generally do not have a pressure sensitivity in a range of less than 10 micro-torr.
A need therefore exists in the prior art for a device for sensing pressure differences having a sensitivity on the order of 1 micro-torr. Hence, a need further exists for a capacitance manometer based sensor capable of distinguishing between pressure differentials on the order of 1 micro-torr and vibrationally induced inertial forces or gravitational forces acting on the diaphragm of the capacitance manometer.